Process for producing oxide coatings on high silicon aluminum alloy



Feb. 6, 1962 J. B. FRANKLIN ETAL 3,020,219

PROCESS FOR PRODUCING OXIDE COATINGS ON HIGH SILICON ALUMINUM ALLOY Filed Jan. 12. 1959 TIME IN MINUTES aavrlo N INVENTORS JOHN B. FQQNKL/N BY O N TAZ- Imam/ca Z -g W ATToz N EYS isoloo-

United States Patent 3,020,219 PRGCESS FOR PRODUCING OXIDE COATINGS N HIGH SILICON ALUMINUM ALLOY John B. Franklin, Norwalk, and Norton W. Vallance, (in, Downey, Califi, assignors, by mesne assignments, to Electralab Printed Electronics Corporation, Needlrarn Heights, Mass, a corporation of Massachusetts Filed Jan. 12, 1959, Ser. No. 786,005 Claims. (Cl. 204-58) This invention relates to the production of hard, dense, smooth and corrosion resistant aluminum oxide films having good dielectric properties, on high silicon bearing aluminum alloys, e.g. die cast or sand cast alloys, by electrolytic oxidation of such alloys. By high silicon aluminum alloys is meant alloys having at least 7% silicon.

In the copending application of John B. Franklin, Serial No. 438,349, filed June 21, 1954 (now Patent No. 2,897,- 125 is disclosed and claimed a process capable of making thicker, denser and harder oxide coatings on aluminum and aluminum alloys than prior art processes, and which also produces thick, dense, hard oxide coatings on aluminum alloys which prior art processes are incapable of coating. Thus, according to the process of such application oxide coating thicknesses up to .010" or more can be obtained. Improved bonding of the oxide coating to the metal is also obtained, and the coating is deposited more rapidly than heretofore in prior art processes.

However, the production of dense hard anodic oxide coatings of any substantial thickness on high silicon aluminum alloys by prior art processes, and including the process of the aforementioned copending application has been difiicult to achieve. Heretofore, acceptable commercial anodized coatings on high silicon aluminum alloys, particularly those containing 7% or more silicon, have not exceeded a thickness of about .001 inch. However, due to the commercial importance of such alloys, processes for successfully forming thick, dense anodized coatings thereon have been heretofore sought without obtaining any commercially feasible solution of the problem.

It is accordingly an object of this invention to produce hard dense electrolytic oxide coatings on high silicon aluminum alloys, including die cast and sand cast alloys, which coatings have a thickness in excess of .001".

Another object is the provision of commercial anodized coatings on aluminum alloys containing at least 7% silicon, said coatings having a thickness greater than .001 and up to and above .005".

A still further object is to afford relatively simple and inexpensive procedure for accomplishing the aforementioned objects in a commercially feasible period of operation.

We have now found that the above objects for producing thick dense oxide coatings greater than .001" thick on high silicon aluminum alloys, can be attained by carrying out the electrolytic oxidation in an electrolyte preferably of relatively high acid concentration equivalent to between about 19% and 25 by volume of 66 Baum sulfuric acid, and increasing the voltage preferably in stepwise voltage increments until a safe maximum voltage rise is obtained without burning. Usually when such voltage rise is attained, an oxide coating of between about .001" and .0015" is formed. The voltage is then maintained substantially constant for an extended period, until the coating thickness builds up to a desired value. In this manner a coating thickness of between about .0015" and about .003" can be obtained. If greater thicknesses are desired, the voltage is increased preferably in stepwise voltage increments to a value described more fully below, and the voltage is then maintained substantially constant again until a desired coating thickness, e.g. up to about a .004" oxide coat is obtained. If still greater coat thicknesses are deice sired, the voltage is raised still higher preferably employing constant voltage increments, and then voltage is maintained substantially constant for an extended period, thus building up an oxide coat thickness of from .005" up to .008".

In preferred practice, during the aforementioned extended constant voltage periods, amperage or current density remains constant during a substantial portion of each period, and decreases during a portion of such periods.

It will thus be seen that the invention provides a 'pro cedure for anodizing high silicon aluminum alloys, which involves one or a plurality of increasing voltage runs, each of which is followed by a substantially constant voltage period for an extended time interval, during a substantial portion of which amperage or current density also remains constant. In this manner unusually thick, dense and hard coatings on the aforementioned alloys are pro duced without the danger of burning.

We do not understand the phenomenon which occurs to produce the above noted results, but we believe that by anodizing the high silicon aluminum alloys, preferably in the aforementioned high concentration of electrolyte, and by maintaining voltage and current substantially constant for an extended period after certain thicknesses of oxide coating have been previously built up by stepwise voltage increases, the oxide coating obtained at commencement of each extended constant voltage period is sufliciently porous to allow further buildup of oxide coating in the acid electrolyte solution, yet is sufiiciently thickand dense to prevent burning or destruction of the oxide coat.

According to the invention, the anodizing operation is carried out in acid concentrations equivalent to from about 7% to 25 preferably 19% to 25%, by volume of 66 Baum sulfuric acid. At the preferred higher acid concentrations within the aforementioned range, thick oxide coats are obtained in relatively short periods of operation. If an acid concentration less than that equivalent to about 19% by volume of 66 Baum sulfuric acid is employed, time of operation is extended for obtaining a given thickness of coating, and if the concentration of acid is greater than that equivalent to 25% by volume of its 66 Baum sulfuric acid, the oxide coating tends to become highly porous and is not commercially acceptable. 5 In preparing the electrolyte, we preferably employ sulfuric acid as the electrolyte, although other strong acids effective for attacking the silicon in thealurninum alloy can be used- Such acids may include sulfamic acid and hydrofluoric acid, for example. These acids are termed electro-anodizing acids herein. Where anodizing acids other than sulfuric acid are employed, a concentration is employed in a range equivalent to that produced by a water solution of 7% to 25 by volume of 66 Baum sulfuric acid,preferably 19% to 5% by volume of 66 Baum:

sulfuric acid, or in other words, equivalent to between 7 and 25 parts by volume of 66 Baum sulfuric acid per parts by volume of total solution.

' We also employ in the electrolyte bath of our process from about 1% to 10%, 6%, by volumeof the aqueous electrolyte solution, of an aqueous extract of peat.

The extract additive can be obtained by extracting peat obtained from various localities, with water. The extraction is particularly made more rapid and yield is increased by extraction at elevated temperature, preferably the atmospheric boiling point of the mixture or more elevated temperature, and most desirably by extraction at elevated temperature under pressure for a period suflicient to produce an aqueous acid solution. The peat used in practiceof 'my invention maybe derived from various locations in the United States, for example, from Georgia, Florida, California or Michigan. This extract additive and its" preferably about 2% to about,

3 mode of preparation are described in Sanford Patent No. 2,743,221.

The part to be coated is connected to the anode of an electrolytic cell and immersed in the electrolyte bath which is maintained at an acid concentration preferably equivalent to between about 19% and most desirably about 19% to 21%, by volume of 66 Baum sulfuric acid, with temperature maintained between about 20 and 50 F., preferably about to 40 F. Direct current for anodizing is applied in the electrolyte. If desired, however, alternating current alone or superimposed on direct current can also be employed.

As the coating becomes thicker, its electrical resistance requires higher voltages for penetration. We have found that the addition of peat extract to the electrolyte is necessary in conjunction with our technique for obtaining the improved results noted herein. The presence of the peat extract aids in preventing burning of the part at high voltages and amperages, and also permits use of low electrolyte temperatures.

In one mode of carrying out our process, the anodizing operation for a high silicon aluminum alloy part is commenced at a voltage of about 20 volts in a sulfuric acid electrolyte containing about 19% to 21% by volume of 66 Baum acid and containing peat extract in an amount of about 3% to 5% by volume, and maintained at a temperature say of about to 45 F. The voltage is in creased in stepwise voltage increments of between 1 and 2 volts up to about 65 to 90, e.g. 70 volts until an oxide coating of between about .001" and .0015 thick is obtained. This may require from about 30 minutes to an hour. Amperage or current density may increase during this period to to 50 amperes per square foot, or higher.

As the voltage is increased at each successive increment of voltage, current rises and then drops back. In following the voltmeter and ammeter during this period, while the current rises and falls in this manner, the needle oi both the voltmeter and ammeter remain quite steady, i.e. without any sudden or rapid fluctuation. As the 65 to 90 volt range is reached, the needle oi the ammeter commences to vibrate. This is substantially the point where the operation is stabilized, since if higher voltage increments are applied beyond this point, the current would be such as to burn the part. This point is thus termed herein the safe voltage rise.

Now voltage is maintained substantially constant at the 65 to 90, e.g. 70, volt value for a period of about 15 to minutes. Amperage is maintained constant for a portion of this period and then decreased in the latter portion of this constant voltage period. As previously indicated, oxide coat builds up during this extended constant voltage period, the coating thickness thus attained ranging from about .0013 to as high as .003" depending on the duration of this period. However, preferably, and particularly where a thickness of coat greater than .002", e.g. a .0025 to .0035" coat, is desired, coating thickness of .0013 to .002" is first attained in the extended constant voltage operation, and the voltage is then raised in 2 to 3 volt per minute constant voltage increments from the 65 to 90 volt value to between about 105 and 130 volts, e.g. 120 to 130, constituting a second safe voltage rise. During the latter period of increasing voltage increments current density ranges from about 20 to 30 amperes per square foot. I

If the desired coating thickness has still not been attained, the voltage and amperage are then again maintained substantially constant for an extended period, that is, voltage is held to say 120 volts and amperage at say 10 to 20 amperes per square foot, for a period of about 30 to 90 minutes, e.g. about 1 hour. During this period, coat thickness can build up to a value of about .0025 to .004".

To obtain coat thicknesses greater than .004", say, .005" to .008", voltage is again increased in voltage increments of about 1 to 2 volts per minute until the desired thickness of coat is produced, or up to about 140 to 160 volts, constituting a third safe voltage rise. At 150 volts a coating thickness of about .005 can be obtained. Voltage and amperage once more can be maintained substantially constant over an extended period, e.g. of about 15 to 120 or more minutes, to obtain coat thicknesses up to .008.

The following are specific examples of operation of our process:

Example 1 A 4" x 4 panel of 13X die cast aluminum alloy containing 12% silicon is anodized in a sulfuric acid electrolyte containing about 20% by volume of 66 Baum sulfuric acid and also containing peat extract in an amount of about 4% by volume of the electrolyte solution. Temperature of the electrolyte is maintained at about 40- 45 F.

The anodizing operation is commenced at 20 volts and the voltage is increased from 20 to 40 volts in 1 volt per minute constant voltage increments (i.e. each voltage increment of one volt maintained constant for one minute), and then increased from 40 to 70 volts in 2 volts per minute constant voltage increments. From 20 to 60 volts current density at the commencement of each voltage increment is at a maximum of about 40 amperes per square foot and decreases to about 30 amperes per square foot at the end of each such increment. From 60 to 70 volts the current density varies as follows from the beginning to the end of each 2 volt per minute constant voltage step.

Current Density at end of Voltage Step (Amps/sq. ft.)

Voltage The oxide coating thickness at the end of the 70 volt increment is .0012".

At 70 volts, voltage is maintained constant for an extended period of 30 minutes. During the first 15 minutes current density remains substantially constant at about 45 amperes per square foot and thereafter gradually decreases to about 20 amperes per square foot at the end of the 30 minute period. A coating thickness of about .0014" is attained at this point.

The voltage is then increased in constant voltage steps of 2 volts per minute from 70 to 120 volts. During this period current density at the commencement of each voltage step increases to about 25 amperes per square foot and decreases gradually during the remainder of each voltage step to about 20 amperes per square foot. At 120 volts a coating thickness of about .00187 is observed.

Voltage is then maintained constant at 120 volts for an extended period of 45 minutes. During the first 30 minutes of this period, current density remains constant at about 20 amperes per square foot, and during the remaining 15 minutes of this period current density decreases gradually to about 15 amperes per square foot. At the end of the 45 minute period, thickness of oxide coat is about .0025". 7

Voltage is then increasedin increments of 1 volt, each increment held constant for one minute, up to a voltage of 145 volts. During each such constant voltage increment, current density increases at the commencement of each such increment to 25 amperes per square foot and then decreases gradually to about 20 amperes per square foot. At 145 volts, a coating thickness of .0043" is attained. Then voltage is maintained constant at 145 volts for minutes. During the first 30 minutes of this period, current density decreases gradually from about 25 to about 10 amperes per square foot, and in the last 60 minutes of this period, current density remains constant at about amperes per square foot. At the end of the 90 minute period, coat thickness is about .006".

The accompanying drawing shows a plot of voltage against time for the entire run of Example 1, particularly pointing out the constant voltage periods at 70 volts, 120 volts, and 145 volts.

Example 2 The procedure of Example 1 is repeated, but using 21% by volume of 66 Baum sulfuric acid, 4% by volume of peat extract, and employing a 319 die cast aluminum alloy containing about 7% silicon. Results similar to Example 1 are obtainable.

Example 3 The procedure of Example 1 is repeated, except that the extended constant voltage periods at 70 volts for 30 minutes, at 120 volts for 45 minutes, and at 145 volts for 90 minutes are omitted. In some instances burning may occur at about 120 volts to produce an unacceptable pitted coating. In those instances where burning does not occur at about 120 volts and the voltage increments are continued to be applied, up to the 145 voltage maximum at the completion of the operation, a coating thickness of only about .001" to .0015" is obtainable at the latter voltage.

While in preferred operation voltage is increased in constant voltage increments during each voltage rise period, the improved results of the invention can also be obtained by increasing the voltage gradually or continuously during each voltage rise period, the rate of such rise being gaged so that it is insuificient to cause burning of the part due to a too rapid increase in voltage causing excessive current flow. Best results are realized, however, When the preferred constant voltage increments are employed during the voltage rise periods before and after the extended constant voltage periods.

Instead of using the aqueous extract of peat as additive in the electrolyte employed in the invention process, other additives may be used which permit the technique and procedure of the invention as described above to be employed. Other suitable additives include, for example, Z-aminoethyl sulfuric acid, taurine, and alkyltaurines, as for example, N-methyl taurine and N-cyclo-hexyl taurine,

and sulfamic acid. The use of the latter compounds as I additives in electrolytes for the electrolytic oxidation of aluminum and its alloys is described and claimed in the Ernst Patents 2,855,350, 2,855,351 and 2,855,352. Such additives should function to permit substantially increasing the voltage in the manner described above without substantial increase in current to a value producing burning, while operating at reduced electrolyte temperatures, to obtain oxide coatings on such high silicon alloys, which are thick, hard and dense, according to the invention. These additives permit such increase in voltage and consequent increase in coating thickness in a shorter period of time than in the absence of such additives, and hence such additives when used in our process are termed oxide coating accelerators. By this term is meant that the additives function to increase rate of growth of oxide coating by permitting increased voltage without producing burning, when employed in conjunction with the other previously described features of our invention including the novel voltage stabilization technique described herein. Other types of silicon alloys of aluminum, which may be die cast, sand cast and the like, containing between 7 and 15% silicon can be employed in our process, in-

' cluding, for example alloys designated 132 13% silicon),

Further, the treatment procedure is relatively simple, and is carried out in conventional and relatively inexpensive electrolytes without undue consumption of electrical energy.

While we have described particular embodiments of our invention for the purpose of illustration, it should be understood that various modifications and adaptations thereof may be made within the spirit of the invention as set forth in the appended claims.

We claim:

1. A process for coating high silicon aluminum alloy articles having at least 7% silicon with a hard and dense coating of oxide of aluminum which comprises passing an electric current at increasing constant voltage increments through an electrolytic cell containing an electrolytewith said article forming the anode, said electrolyte maintained between about 20 and 50 F. and comprising a water solution of between about 19% and 25% by volume of 66 Baum sulfuric acid and an aqueous peat extract as additive, interrupting the voltage rise at between about 65 and volts when an oxide coating of between about .001 and .0015" thick is formed, and maintaining the voltage substantially constant for a period of about 15 to 45 minutes at said interrupted voltage value to increase the oxide coating thickness during said period.

2. A process as defined in claim 1, wherein voltage is increased by a plurality of constant voltage increments above the voltage of said constant voltage period, and forming additional oxide coating until a second voltage rise is obtained, said last mentioned voltage rise is interrupted between about and volts, and the voltage is again maintained substantially constant for a period of about 30 to 90 minutes at said last mentioned interrupted voltage value, to increase the oxide coating thickness during said last mentioned period.

3. A process for coating high silicon aluminum alloy articles having a silicon content of from about 7% to 15 by weight of silicon, .with a hard and dense coating of oxide of aluminum, which comprises passing an electric current through an electrolytic cell containingan electrolyte maintained between about 20 and 50 F. with said article forming the anode, said electrolyte comprising a water solution of about 19% to about 25 by volume of solution of 66 Baurn sulfuric acid, and 1 to 10% by volume of said solution of an aqueous extract of peat, raising the voltage to about 65 to about 90 volts in stepwise constant voltage increments, until an oxide coating between about .001" and about .0015 thick is formed, maintaining the voltage substantially at said last-mentioned voltage value for a period of about 15 to 45 minutes to increase the oxide coating thickness during said period, raising the voltage in stepwise constant voltage increments to about 105 to 130 volts, and forming an oxide coating having a thickness greater than the aforementioned thickness.

4. A process for coating high silicon aluminum alloy articles having a silicon content of from about 7% to 15 by weight of silicon, with a hard and dense coating of oxide of aluminum which comprises passing an electric current through an electrolytic cell containing an electrolyte maintained between about 20 and 50 F. with said article forming the anode, said electrolyte comprising a water solution of about 19% to about 25 by volume of solution of 66 Baum sulfuric acid, and 1 to 10% by volume of said solution of an aqueous extract of peat, raising the voltage from about 20 volts to from about 65 to about 90 volts in stepwise constant voltage increments 7 of about 30 to 90 minutes until an oxide coating of .0025" to .004" is formed.

5. A process for coating high silicon aluminum alloy articles having a silicon content of from about 7% to about by weight of silicon, with a hard and dense coating of oxide of aluminum which comprises passing an electric current through an electrolytic cell containing an electrolyte maintained between about and F. with said article forming the anode, said electrolyte comprising a water solution of about 19% to about 21% by volume of solution of 66 Baum sulfuric acid, and 2% to 6% by volume of said solution of an aqueous extract of peat, raising the voltage from about 20 volts to from about to about volts in stepwise constant voltage increments of about 1 to 2 volts per minute until an oxide coating between about .001" and about .0015" thick isformed, maintaining the voltage substantially constant at said lastmentioned voltage value for a substantial period of about 15 to 45 minutes, raising the voltage in stepwise constant voltage increments of about 2 to 3 volts per minute to about to volts, and maintaining the voltage sub stantially constant at said last-mentioned voltage value for a second substantial period of about 30 to 90 minutes, until an oxide coating of .0025" to .004" is formed.

6. A process as defined in claim 5, wherein voltage is increased in stepwise constant voltage increments of about 1 to 2 volts per minute to about to volts, and voltage is maintained substantially constant at said last mentioned voltage value for an additional extended period of about 15 to 120 minutes until an oxide coating of .005 to .008" is formed.

7. A process for coating high silicon aluminum alloy articles having at least 7% silicon with a hard and dense coating of oxide of aluminum which comprises passing an electric current at increasing voltage at a rate insuflicient to cause burning of said article, through an electrolytic cell containing an electrolyte with said article forming the anode, said electrolyte comprising a water solution of between about 7% and 25% by volume of 66 Baum sulfuric acid and an oxide, coating accelerator as additiveuntil a voltage between about 65 and 90 volts is reached and an oxide coating of between about .001" and .0015" thick is formed, and maintaining the voltage substantially constant at said last mentioned voltage value for a period of about 15 to 45 minutes to increase the oxide coating thickness during said period.

8. The process as defined in claim 7, wherein said electrolyte is maintained at a temperature between about 20 and 50 F.

9. A process for coating high silicon aluminum alloy articles having at least 7% silicon with a hard and dense coating of oxide of aluminum, which comprises passing an electric current at constant voltage increments through an electrolytic cell containing an electrolyte with said article forming the anode, said electrolyte maintained between about 20" and 50 F. and comprising a water solution of between about 19% and 25% by volume of 66 Baum sulfuric acid and an oxide coating accelerator as additive until a voltage of between about 65 and 90 volts is reached and an oxide coat of between about .001" and .0015" thick is formed, and maintaining the voltage substantially constant at said last mentioned voltage value for a period of about 15 to 45 minutes to increase the oxide coating thickness during said period.

10. A process as defined in claim 9, including in said electrolyte an aqueous extract of peat.

References Cited in the tile of this patent UNITED STATES' PATENTS 1,771,910 Bengough et al July 29, 1930 2,111,377 Wales Mar. 15, 1938 2,743,221 Sanford Apr. 24, 1956 2,855,352 Ernst Oct. 7, 1958 2,897,125 Franklin July 28, 1959 2,918,416 Taylor Dec. 22, 1959 FOREIGN PATENTS 716,554 Great Britain Oct. 6, 1954 

1. A PROCESS FOR COATING HIGH SILICON ALUMINUM ALLOY ARTICLES HAVING AT LEAST 7% SILICON WITH A HARD AND DENSE COATING OF OXIDE OF ALUMINUM WHICH COMPRISES PASSING AN ELECTRIC CURRENT AT INCREASING CONSTANT VOLTAGE INCREMENTS THROUGH AN ELECTROLYTIC CELL CONTAINING AN ELECTROLYTE WITH SAID ARTICLE FORMING THE ANODE, SAID ELECTROLYTE MAINTAINED BETWEEN ABOUT 20* AND 50* F. AND COMPRISING A WATER SOLUTION OF BETWEEN ABOUT 19% AND 25% BY VOLUME OF 66 BAUME SULFURIC ACID AND AN AQUEOUS PEAT EXTRACT AS ADDITIVE, INTERRUPTING THE VOLTAGE RISE AT BETWEEN ABOUT 65 AND 90 VOLTS WHEN AN OXIDE COATING OF BETWEEN ABOUT .001" AND .0015" THICK IS FORMED, AND MAINTIANING THE VOLTAGE SUSBTANTIALLY CONSTANT FOR A PERIOD OF ABOUT 15 TO 45 MINUTES AT SAID INTERRUPTED VOLTAGE VALUE TO INCREASE THE OXIDE COATING THICKNESS DURING SAID PERIOD. 